The subject matter of this application relates to systems and methods that provide optical signals over a cable transmission network.
Early CATV systems were configured to deliver content along a coaxial transmission path from a content provider to its subscribers. As these systems evolved to not only provide a greater amount of content, but to provide data services that travel to and from the subscriber (e.g. Internet service), much of the coaxial path from the provider to the subscribers was replaced with fiber-optic cable, which could transmit a signal over a greater distance with less intermediate amplification. Initially, 1310 nm fiber-optics were deployed, but as CATV systems continue to evolve, much of the 1310 nm infrastructure is being upgraded with 1550 nm fiber optic infrastructure, as the 1550 nm wavelength reduces attenuation losses along the length of the cable, and allows the use of wavelength division multiplexing (WDM) to simultaneously transmit both analog and digital signals.
Due to the combination of laser chirp with fiber dispersion, a directly modulated laser operated in the 1550 nm wavelength tends to produce a great deal of distortion in the CATV transmission system. Therefore, indirectly modulated lasers have been used to emit a signal onto 1550 nm fiber optic cable. Generally speaking, there are two known approaches to dispersion compensation, one being optical dispersion compensation and the other being electronic dispersion compensation. Optical dispersion technologies are expensive, prone to optical losses, and can often incur some undesired signal degradations, such as filter band limitation and optical non-linearities. Electronic dispersion technology on the other hand is less expensive relative to optical modulation approaches, but has disadvantages such as its limitation in dispersion compensation, which limits the number of analog channels to be transmitted and transmission distance. Therefore, improvement in the dispersion compensation capability is needed.